Glass-free silicon carbide ceramics are known for maintaining their strength at high temperatures. They are made by sintering silicon carbide powder containing boron and carbon additives at temperatures in excess of 2000.degree. C., which, in contrast to the majority of other ceramics, results in a dense ceramic that contains no amorphous or glassy phases between the grains. Because SiC is a covalently bonded material, it is difficult to sinter by solid state diffusion processes. The addition of boron and carbon has been found to enhance the surface diffusion of the grains, which allows the material to sinter, although at very high temperatures.
Recently, attempts have been made to liquid-phase sinter SiC at lower temperatures by using oxide additives such as Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3. The goal has been to add these oxides in a ratio that produces YAG (yttrium aluminum garnet) upon cooldown from sintering in order to obtain a totally crystalline product. A material that has been commercialized using this method is Carborundum HEXALOY-SX. However, the oxide sintering aids react with the surface of the silicon carbide to yield a silicate glass. This glass is difficult to crystallize, and a thin amorphous layer remains between the grains. Consequently, the major problem with this approach is that the excellent high temperature properties of SiC are compromised due to the presence of a glassy film that results from the addition of the oxides.